Eye tracking systems forming part of human-machine interfaces (HMIs) are subject to severe restrictions on power consumption, especially in personal computers and battery-powered devices. Many available eye tracking systems are programmed to enter a sleep mode after a period of no input from a user. From the point of view of energy economy, the period should be as short as possible, so that the eye tracking system will be in a sleep mode a large percentage of the time. This must be balanced against the desirability of high responsiveness of the system, which would seem to advocate a relatively longer period to elapse before sleep mode is entered. This tradeoff may become less awkward, however, if efforts are directed at reducing the wake-up time of the system.